MOCVD法生长ZnO薄膜的结构及光学特性

Structure and Optical Property of ZnO Thin Films Grown by MOCVD

采用MOCVD方法在c Al2 O3衬底上生长出了具有单一c轴取向的ZnO薄膜 ,采用X射线衍射 (XRD)、Raman散射、X射线光电子能谱 (XPS)及光致发光 (PL)谱等方法对ZnO薄膜的结构及光学特性进行分析测试。XRD分析只观察到ZnO薄膜 (0 0 0 2 )衍射峰 ,其FWHM数值为 0 1 84°。Raman散射谱中 ,4 35 32cm- 1 处喇曼峰为ZnO的E2 (high)振动模 ,A1 (LO)振动模位于 5 75 32cm- 1 处。XPS分析表明 :ZnO薄膜表面易吸附游离态氧 ,刻蚀后ZnO薄膜O1s光电子能谱峰位于 5 30 2eV ,更接近Zn—O键中O1s电子结合能 (5 30 4eV)。PL谱中 ,在3 2 8eV处观察到了自由激子发射峰 ,而深能级跃迁峰位于 2 5 5eV ,二者峰强比值为 4 0∶1 。

ZnO, a Ⅱ Ⅵ compound semiconductor with a wide direct band gap of 3 37 eV at room temperature, is a multi functional material which can be used in varistors, phosphors, gas sensors, transparent conductive thin films and surface acoustic wave devices, etc . We prepared ZnO thin films on c plane sapphire substrates by metal organic chemical vapor deposition and investigated thoroughly the structure and optical properties by X ray diffraction (XRD), Raman spectrum, X ray photoelectron spectrum (XPS) and photoluminescence (PL) spectrum. ZnO thin films were fabricated on (0001) sapphire at optimized temperature of 580 ℃ under the reactor pressure of 480 Pa by using a low pressure MOCVD apparatus. Diethylzinc (DEZn) and oxygen as the reactants were introduced into the reactor separately. The structure and crystallinity of the ZnO thin films were characterized by XRD. Only (0002) diffraction peak of ZnO with hexagonal wurtzite structure could be observed. The diffraction peak positioned at 2 θ =34 48° and the value of the full width at half maximum (FWHM) was 0 184°, which implied a good crystalline quality of ZnO thin films. The lattice constant calculated were 0 326 1 nm and 0 519 8 nm for a and c axes respectively. It could be concluded that the tensile stress along the a axis exist in the ZnO thin film. The averaged grain size was 47 12 nm. Raman scattering spectrum of the ZnO thin films was performed at room temperature. E 2 (high) mode and A 1 (LO) mode were at 435 32 cm -1 and 575 32 cm -1 respectively. The former corresponded to the band characteristic of wurtzite phase, while the latter was related to the formation of oxygen deficiency, interstitial Zn and free carrier. Both modes shifted to high frequency side due to the tensile stress along the a axis. In addition, X ray photoelectron spectrum showed that O 2 could adsorb in the surface of the sample. After etching for 10 minutes, the quantity of O 2 adsorbed was reduced and the binding energy of O 1s was 530 2 eV, which was close to that in Zn-O bond. Simultaneously, the PL spectrum of the ZnO thin film was measured. A strong near band edge (NBE) emission was obviously observed, which was at 3 28 eV with the FWHM of 125 meV. The deep level emission (DLE) centered on 2 55 eV was quite weak. The ratio of the intensity of the NBE to that of DLE was 40∶1, which suggested better optical quality of the ZnO film grown by MOCVD at the present growth conditions.